1. Field of the Invention
The present invention relates to a pedal simulator as a cylinder mechanism including a plurality of disc springs for normally urging a piston toward a fluid chamber for introducing a working fluid thereinto, the fluid chamber being defined partly by the piston, and also including a housing in which the piston is slidably held, wherein the housing has an open end on an opposite side of the housing from the piston and a lid member which covers the open end and is separate and distinct from the housing.
2. Description of the Related Art
In recent years, there has been proposed a brake apparatus for electrically actuating a motor cylinder or a fluid pressure pump to apply a fluid pressure to a braking force generator mounted on a wheel, based on operating information such as of a stepping force on a brake pedal (hereinafter simply referred to as “pedal”) which is supplied to a vehicle ECU.
Such a system for electronically controlling brakes with an electric signal, or in other words, a brake-by-wire system, makes it possible to apply braking forces more responsively to the pedal operation and to brake the vehicle more smoothly than the fluid pressure control brake system that has widely been used heretofore which includes a pedal and a master cylinder directly connected to each other.
The brake-by-wire brake apparatus includes a pedal simulator (stroke simulator) for producing a reactive force in response to the depression of the pedal. Owing to the pedal simulator, the depth to which the pedal is depressed (stroke or controlled variable) and the stepping force are related to each other according to certain characteristics (stroke vs. stepping force characteristics).
Japanese Laid-Open Patent Publication No. 06-211124 discloses a cylinder mechanism, serving as such a pedal simulator, which comprises a piston defining a fluid chamber for introducing a working fluid thereinto and a plurality of disc springs for normally urging the piston into the fluid chamber. Based on the characteristics of the disc springs, nonlinear stroke vs. stepping force characteristics are provided to reduce the pedal operating torque (stepping-resistant rigidity) when the pedal starts being depressed and to increase the pedal operating torque as the pedal is progressively depressed, thereby giving the driver a natural operational feeling in operating the pedal.
In the disclosed cylinder mechanism, most of the disc springs are stacked together while facing in the same direction. In order for the piston to slide over a sufficient stroke, the cylinder mechanism needs to have a considerable number of disc springs and hence is costly to manufacture.
One solution to reduce the number of disc springs in the cylinder mechanism is shown in FIG. 9A of the accompanying drawings. According to the solution shown in FIG. 9A, the disc springs have respective bottom ends facing together in a housing 100. Specifically, disc springs 102 have upwardly directed conical shapes and disc springs 104 have downwardly directed (reversely directed) conical shapes. The structure shown in FIG. 9A is effective to reduce the number of stacked disc springs and to allow the piston 106 to slide over a sufficient stroke.
Normally, slight clearances C are required between the disc springs 102, 104 and the housing 100; and also between the disc springs 102, 104 and a shaft 108 around which the disc springs 102, 104 are disposed, in view of the flexure of the disc springs 102, 104, as shown in FIG. 9A. As shown in FIG. 9B of the accompanying drawings, when a piston 106 on the disc springs 102, 104 is pushed, the disc springs 102, 104 are compressed. Thereafter, as shown in FIG. 9C, when the piston 106 is retracted and the disc springs 102, 104 are expanded, the disc springs 102, 104 may possibly be diametrically displaced across the clearances C, as indicated by A in FIG. 9C of the accompanying drawings.
If the disc springs 102, 104 are displaced as indicated by A, then the disc springs 102, 104 tend to suffer irregular sliding resistance each time they are expanded and compressed. Therefore, the stroke vs. stepping force characteristics undergo complex changes each time the disc springs 102, 104 are compressed, and become unstable. The simple face-to-face placement of the disc springs 102, 104 makes it difficult to manage or hold them in a stable stacked state, and results in change in the spring characteristics of the disc springs 102, 104 each time they are expanded and compressed. The changing spring characteristics cause unwanted differences between individual cylinder mechanisms.